The working mechanism of a liquid crystal display panel is as follows: orientation of liquid crystal molecules sandwiched between opposite transparent electrodes are controlled by voltages applied to the opposite transparent electrodes, so that the amount of light which enters liquid crystal molecules via one electrode and leaves the display panel via the other electrode is controlled; the more the amount of transmitted light is, the higher the brightness will be.
As for a liquid crystal display panel, a nonlinear relationship presents between the applied voltage (i.e. the voltage applied to the transparent electrodes) and light transmittance (grayscale) achieving the best display effect. A curve drawn according to this relationship is referred to as an idea gamma curve. Due to the nonlinear relationship between the applied voltage and the grayscale, a certain increment in the applied voltage does not result in the same increment in the grayscale. Thus, when the liquid crystal display panel is controlled by the applied voltages whose values linearly correspond to the brightness of input images to output images, the output images may be brighter or darker (distorted) as compared to the input images. Therefore, it is necessary to correct the voltages applied to the electrodes according to the idea gamma curve of the liquid crystal display panel so as to avoid distortion of output images. This process of correction is referred to as gamma correction.
A gamma correction circuit is required to perform gamma correction on a liquid crystal display panel. The gamma correction circuit comprises gamma registers and D/A converting units. Data of input images is stored in the gamma registers after converted to digital signals, and is then D/A converted by the D/A converting units to generate gamma voltages corresponding to respective grayscales, and the gamma voltages are finally output to data lines of pixel units.
In the process of operating the above gamma correction circuit, a curve drawn according to correspondence between the gamma voltages generated after D/A conversion and grayscales is referred to as a test gamma curve. Since a gamma correction circuit is designed according to the idea gamma curve, the test gamma curve should coincide with the idea gamma curve. However, due to the limitation of production process, for some batches of display panels of the same model, the idea gamma curve may change. If these display panels use the same gamma correction circuit as the other batches of display panels, the corresponding test gamma curve will not change, which results in a large deviation between the test gamma curve and the changed ideal gamma curve and further leads to display distortion of a part of display panels of the same model.